The majority of X-ray apparatuses currently used in hospital wards for diagnosis of non-transferable hospitalized patients are capacitor-type X-ray apparatuses. This type known X-ray apparatus, however, has problems because an electric cord for the connection of the X-ray apparatus to a power source has to be connected to the terminals of an ac power source such as the plugs provided in each patient room for each use, and, owing to the small amount of X-rays produced, it is not possible to take X-ray pictures of a thick object such as the lumbar, thus limiting the regions which can be examined. Such an X-ray apparatus used for visiting rounds in a hospital must solve these problems, and at the same time it must satisfy the following essential requirements of an X-ray apparatus intended for visiting rounds in a hospital: (1) the X-ray emission operation must be simple and the functions of the control circuit must be stable and (2) the apparatus must be light in weight to facilitate the visiting rounds to the patients' rooms in the hospital wards. In order to satisfy these conditions and requirements, there is a method comprising converting the dc voltage from a storage cell included in the apparatus to an ac voltage and raising this voltage to the higher voltage required for the emission of X-rays. However, the control circuit for converting the dc voltage to an ac voltage has to be simple in structure and well stabilized to achieve the foregoing purposes.
In general, the voltage across the terminals of a storage cell will drop progressively as this storage cell is progressively discharged. That is, as compared with the terminal voltage when the storage cell is fully charged, the terminal voltage of the storage cell when this cell must be re-charged will have a value reduced by about 20 to 30%. The amount of X-rays produced varies in proportion to 2 to 5 times the square of the X-ray tube voltage. From this point of view it is known that, in a transformer type X-ray apparatus the range of permissible difference in the X-ray tube voltage is considered to be .+-.7%. This limitation is much more severe than the permissible difference for the X-ray tube current or for the time of photography. Accordingly, when a storage cell is used as the power source for an X-ray apparatus, it is necessary to effect compensation, by some means or other, this drop in the terminal voltage of the storage cell and to minimize the variation in the utilizable amount of X-rays caused by the changes in the X-ray tube voltage. Furthermore, as the discharge of the storage cell progresses and as its terminal voltage reaches a level at which re-charging of the storage cell is inevitable, it is necessary to stop further discharge for protection of the storage cell. In known storage cell type X-ray apparatuses, there is adopted a method of manually regulating the positions of the slidable contacts of a transformer employed for regulation of the voltage in order to compensate for the drop in the X-ray tube voltage caused by the voltage drop of the storage cell. This manual operation method, however, requires constant monitoring of the voltage level of the storage cell, so that the burden imposed on the operator is heavy, and what is more, there is the fear that, if this manual operation is neglected, the storage cell will be excessively discharged so that the life of this storage cell could be shortened. If, as a countermeasure, a storage cell having a very large capacity is used, this will inevitably lead to an increase in the weight and the size of the storage cell, and accordingly the price of the apparatus as a whole would be higher.
In a storage cell type X-ray apparatus, a rectangular wave voltage is generally used. This rectangular wave voltage is such that the built-up of voltage at the time of turning-on is quick, and accordingly, an abnormally high voltage may be developed due to transitional phenomenon. Thus there is the danger that the X-ray tube may be broken and/or that a dielectric breakdown may occur. In the conventional X-ray apparatus using the commercial power source, the connection of the apparatus to the power source occurs at zero phase in order to prevent the generation of an abnormally high voltage at the time of starting the X-ray emission. However, in an X-ray apparatus of the dc-ac conversion type such as described above, it should be understood that the formation of an input waveshape such as a sine wave having a slow built-up in order to achieve connection of the X-ray apparatus to the power source at zero phase involves the problem that the circuitry required tends to become too complicated and too large in size when consideration is given to the fact that the power source device of the X-ray apparatus is designed to use momentary on-off pulses having large current. However, in an X-ray apparatus of the type in which the high voltage power supply device and the X-ray tube device are used in the vicinity of their rated voltage, it is mandatory that any abnormally high voltage at the time of the start of emission X-ray (at the time that an electric voltage is connected to the apparatus) be unfailingly controlled from view point of protection of the apparatus and safety.